Improving the accuracy of a discrete repetitive controller applied to an active power filter when the mains frequency is unknown

Abstract

Repetitive controllers are used to track periodic references or to reject periodic disturbances, exactly. When implementing this type of controllers digitally, the sampling period has to be chosen so that there is an integer number of samples within a period of the signal to be tracked or rejected. Otherwise, the controller performance deteriorates dramatically. This requirement is not always easy to fulfil when the signal period is not known exactly at the design stage or when it changes during operation. This paper presents a simple addition for a first-order discrete repetitive controller to be used in applications in which the period of the reference signal or disturbance is not an exact multiple of the sampling period. The algorithm is proposed to tackle the problem of small errors in the mains frequency estimation for an active power filter to be used in an electrical-energy distribution system. The paper illustrates how the results of a previously-tested active power filter are improved with the addition proposed.